The present invention is directed to removing paint from large substrates or structures, such as aircraft, buildings, metallic locks and dam gates, by pyrolysis using microwave coupling compounds which are sprayed on the structure.
Several conventional methods of paint removal are used, such as mechanical scraping/grinding, abrasives, blasting, blow torching, etc. These methods are, however, used on buildings and are labor intensive. In older housing structures lead based paint has been used which needs to be removed safely.
In addition, chemical strippers containing methyline chloride, phenol and formic acid are effective in paint removal in the metal finishing industry. However, the use of toxic organics is limited by EPA regulations to 2.13 mg/1 of waste effluent, and methylene chloride and phenol are recognized as hazardous.
Fluidized bed paint stripping and sludge burning is accomplished by using a fluid bed furnace heated to 800 degrees F. to pyrolyze the paint. The fluidized bed furnace is filled with aluminum oxide as the fluidized medium. Items to be stripped are suspended in a loading frame on a basket. The bed is fluidized using air and 150 mesh solids of Al.sub.2 O.sub.3. The load of painted articles is typically held in the fluidized furnace for 30 minutes. Under these conditions, most of the organic material is pyrolyzed to hydrocarbon gas because there is insufficient oxygen available for combustion. This method is not effective for large structures because it involves immersion of the part in the furnace. The size of the part to be stripped is therefore limited by the size of the furnace.
Another process for removing carbonizable adherent coating on the surface of metal parts is heating the part to carbonizing temperatures and blasted with heated blasting agents. Then the parts are cooled in liquid nitrogen to cause embrittlement of the carbonized coating. This process is used to remove paint from electric cables and from hangers for automobile printing. This process is also limited by the size of the retort and can not be used on large structures.
Aircraft are painted for several reasons, such as,
a. Protection from corrosive atmospheres;
b. Aesthetic reasons as well as informational record;
c. Camouflage;
d. Radar suppression; and
e. IR suppression.
Some of these aircraft must be stripped of paint and repainted every two to three years for,
a. Inspection of the extent of corrosion;
b. Paint damage repair;
c. Change of top coat systems; and
d. Removal of weathered paint.
For some aircraft structures made from polymeric composites, such as helicopter rotors, hand or power tool scraping is required because of close tolerances.
Stripping of old paint from an aircraft is accomplished by coating the aircraft with a chemical stripping compound and scrubbing it off. This method yields a hazardous sludge which must be sealed in barrels and then buried in landfills. The increased difficulty in finding landfill areas as well as the large cost associated therewith contributes to high cost of paint stripping.